Figure 12.

Rate of isotonic transport varies with solute flux through cells (see also Fig. 10), here accomplished by varying pump density on lateral intercellular space membrane (under conditions of near-constant intracellular concentration of driving solute, not shown). Water fluxes shown here span the range obtained with amphibian leaky epithelia (few nl·s⁻¹· cm⁻²) to the very active rat proximal tubule (40–60 nl·s⁻¹· cm⁻²). Inset: If the major component of water uptake takes place via tight junction, the energetic efficiency is high. With mitochondrial P/O ratio of 3, an efficiency of, J ^NET/J ^PUMP = 1.5, would correspond to 27 moles Na⁺ transported for each mole of O₂ consumed by activity of sodium pump in accord with mammalian kidney (see Discussion). The model can produce this result with transjunctional hydraulic conductance (L ^tm) being an order of magnitude larger than the translateral hydraulic conductance [L ^tl = L ^a ·L ^lm /(L ^a + L ^lm )].